Menstrual Cycle: How it works, the mystery revealed.

I get a lot of questions about how the menstrual cycle works, what is a normal cycle, what causes irregular cycles and how birth control pills work. The next few posts will tackle these questions. Today: The normal menstrual cycle. As you will see, normal has significant variations.


The menstrual cycle is the basis of our ability to reproduce. Just as it is so easy to forget that eating exists primarily to provide nutrients for our body’s health and survival, so too is it easy to forget that our* menstrual cycles exist in order to make an egg ready to join with a sperm and prepare the uterine lining to accept the fertilized egg.

For an illustration of the reproductive organs and a helpful website check out: American Sexual Health Association

Menses begin with puberty and menarche (the medical term for the onset of menses) and continues until menopause (beginning one year after your last menses). Most women have many, many menstrual cycles in our lives. For many women, much of our reproductive lives are focused on how we can safely control our fertility and have our children when and if we want. Understanding the menstrual cycle is a key to understanding how our bodies work, how contraception works and how pregnancy is achieved.

Day one of the menstrual cycle is the day that bleeding (menses) starts. A typical cycle –from start of the menses, until the start of the next menses ranges from 25-36 days, although cycles as short as 21 days are still considered normal. Only small percentage of women has the classic 28-day cycle.

Bleeding lasts from 3-7 days, with a total blood loss of 15-80 milliliter (ml), which is equal to .5-2.5 ounces. A regular tampon holds 5 ml of blood; a super tampon or maxi-pad holds 10 ml. Menstrual cups vary but some have volume markers. Small blood clots do occur at the higher levels of blood loss, and are normal up to about the size of a dime.

Premenstrual symptoms of mild breast tenderness, mood changes and food cravings are common. Menstrual cramps happen and typically go away with time, heat, exercise, herbs or ibuprofen. As long as these symptoms are mild, there is no need for concern or treatment.

The cycle phases

There are three phases to the menstrual cycle: follicular, ovulatory and luteal. I will give a brief explanation of these followed by a more technical one, for those interested.

The basics: Follicular phase

This phase begins with day one of the cycle—when you start bleeding. At this point the two main reproductive hormones, estrogen and progesterone are at their lowest levels.

The low levels of these hormones signal the brain to signal the ovary to grow follicles-up to 30 of them. Inside each follicle is an egg. Usually only one follicle gets fully mature. The remaining follicles dissolve.

The dominant follicle causes an increase to the level of estrogen and progesterone. As a result the lining of the uterus begins to grow as well.

Ovulatory phase

Usually about two weeks before the end of the cycle, the brain releases a hormone, which causes the one mature follicle to break open, or rupture and release the egg. This is ovulation.

The egg travels down the fallopian tube. If there are sperm there, this is typically where the egg gets fertilized. It begins to divide creating an embryo this will eventually become the baby.

Luteal phase

After the egg leaves the ovary, the follicle that held the egg evolves. It becomes the corpus luteum and creates more progesterone. The production of estrogen gets slightly less. This hormonal change helps prepare the lining of the uterus to either receive the fertilized egg, or to shed it in an organized fashion.

If no egg is fertilized, the estrogen and progesterone production drops and the uterine lining that built up over the month begins to come out. This brings us back to day one of the cycle—the menses.

For those who are interested in the same story with more detail:

The regulation of the female reproductive cycle is referred to as the HPO axis or the Hypothalamic-Pituitary-Ovarian Axis. The hypothalamus and pituitary are parts of the brain. The HPO axis refers to hormonal messenger communication between these three structures: the hypothalamus, pituitary and ovaries. This communication results in timing of the menstrual cycle, the selection and maturation of a follicle and egg and the preparation of the uterus, breast and vagina in the event of conception. In the absence of conception a different combination of hormones then cause the uterus to release the lining that was built up during the month, creating the menses.

The follicular phase

On day one of the cycle, when the menses start, estrogen and progesterone are at their lowest levels. This lack of hormones signals the hypothamus to secrete GnRH (gonandotropin releaseing hormone). GnRH acts on the pituitary gland causing it to secrete FSH (follicle stimulating hormone) and LH (luteinizing hormone), which act on the ovary.

The ovaries respond by activating up to 30 follicles. Each follicle contains an egg. By the time a female fetus is 5 months old, she has all the eggs created that she will have in her life; they exist in these follicles. Most follicles/eggs will reabsorb before activating a menstrual cycle. Of the up to 30 that activate monthly, typically only one matures. Occasionally a second follicle survives and if both eggs are fertilized you have twins that are not identical. Identical twins occur when one egg and one sperm join and then that entity splits in half creating two genetically identical people.

Back to the follicle:

The growing follicle increases the production of estrogen and progesterone. These hormones act on the inside lining of the uterus causing it to grow. There are also changes in the breasts and vagina due to these hormones. The increase in estrogen also causes the hormone LH (luteinizing hormone) to rise.

Ovulatory phase

As estrogen levels peak, a surge of LH is released which causes the mature follicle to rupture– this is ovulation. This generally occurs 16-32 hours after the LH peak (ovulation predictor tests are testing for this LH peak). The egg is released from the follicle and is then pulled into the fallopian tube by little cilia (hair like projections) which also move the egg along and into the uterus. If sperm are present in the fallopian tube, conception may occur.

Luteal Phase

After it ruptures the follicle transforms into the corpus luteum. This is the most consistent phase of the cycle, lasting 14 days—timed from ovulation to menses. So if you have a 34 day cycle, the last 14 days are the luteal phase, the variation in cycle length generally occurs in the follicular phase.

The corpus luteum secretes progesterone, which acts on the lining of the uterus. It prepares it for either implantation of the fertilized egg or for orderly shedding.

Progesterone is thermogenic—meaning it causes the body temperature to increase –by about .9 degree F.

If conception occurs, the corpus luteum continues to produce progesterone under the influence of a pregnancy related hormone –HCG human chorionic gonadotropin (this is what pregnancy tests check for). The corpus luteum is active until the placenta grows and takes over the support role.

In the absence of fertilization and implantation, the corpus lutem devolves and becomes inactive. Additionally, the hormones estrogen and progesterone drop rapidly. This hormonal shift causes the uterine lining to shed and the menses occur and we are back at day one.

That is the menstrual cycle. I plan to follow this blog with others related to: How hormonal contraception works, common variation on the normal cycle: irregular timing, heavy cramping, heavy bleeding and such. Stay tuned.


*I write to my audience as female. For my male readers, I welcome you, and offer you the opportunity to experience a common female reality–one of translating gender pronouns in order to feel included.